In general, a power transmission device is used to selectively transmit power of one member to another member. The power transmission device is also used to stop a rotating member.
Referring to FIG. 11, a conventional power transmission device for a tractor will be briefly described.
As shown in FIG. 11, two power transmission devices 200 are disposed on one input shaft (not shown). Each power transmission device 200 includes a housing that forms a mounting space and shares at least a portion of the housing with the other power transmission device 200. That is, each housing includes a clutch hub 210, a clutch connecting portion 212, a clutch housing 214 and a disk housing 215, and the two housings share the clutch connecting portion 212 in common.
Each power transmission device 200 further includes a piston 230, first and second friction disks 250 and 252, a return spring 240 and a power transmission hub 260.
The piston 230 is mounted in the housing and forms a piston chamber 220 between the housing and the piston 230. If an operating hydraulic pressure is supplied into the piston chamber 220, the piston 230 can move in the housing in an axial direction. In addition, an operating hydraulic pressure supply line 216 is formed in the clutch hub 210 or the clutch connecting portion 212 to supply the operating hydraulic pressure into the piston chamber 220. Sealing members 270 and 272 are mounted between a piston interior circumferential portion 232 and the clutch hub 210 and between a piston exterior circumferential portion 236 and the clutch housing 214 to prevent an operation oil that is supplied into the piston chamber 220 from leaking from the piston chamber 220. In addition, the piston exterior circumferential portion 236 and a piston middle portion 234 are protruded in an opposite direction to the piston chamber 220 to form two pushing portions.
The disk housing 215 extends from the clutch housing 214 in the axial direction and a plurality of first friction disks 250 are splined to an interior circumference of the disk housing 215. In addition, a plurality of second friction disks 252 are splined to an exterior circumference of the power transmission hub 260 and are alternately disposed with the plurality of first friction disks 250. In addition, a supporting plate 254 is splined to the disk housing 215 and movement of the supporting plate 254 in the axial direction is restricted by a snap ring 256 mounted at the disk housing 215. The supporting plate 254 supports the first friction disks 250 and the second friction disks 252 in the axial direction to be frictionally coupled to each other.
A spring supporting member 242 is disposed on the clutch hub 210 at an opposite side to the piston chamber 220 and a return spring 240 is disposed between the piston interior circumferential portion 232 and the spring supporting member 242. Movement of the spring supporting member 242 in the axial direction is restricted by a snap ring 244 mounted on the clutch hub 210. Therefore, the return spring 240 exerts a spring load counteracting against an axial force generated by the operating hydraulic pressure on the piston 230. In addition, passages 218 and 262 through which a cooling oil is supplied are formed in the clutch hub 210 and the power transmission hub 260.
According to the power transmission device 200, if the operation oil is supplied into the piston chamber 220 through the operating hydraulic pressure supply line 216, the piston 230 moves to one side in the axial direction and frictionally couples the first friction disk 250 and the second friction disk 252. Therefore, rotational power of the input shaft is transmitted to the power transmission hub 260. On the contrary, if the operation oil supplied into the piston chamber 220 is exhausted from the piston chamber 220, the piston 230 moves to the other side in the axial direction by the spring load of the return spring 240. Therefore, the first friction disk 250 and the second friction disk 252 are spaced from each other and the rotational power of the input shaft is not transmitted to the power transmission hub 260.
According to the conventional power transmission device 200, the return spring 240 is disposed between the power transmission hub 260 and the clutch hub 210 in a radial direction. Therefore, area of the first friction disks 250 and the second friction disks 252 cannot be sufficiently increased if an exterior diameter of the housing is not increased. If the area of the first friction disks 250 and the second friction disks 252 are increased, a mass of a core plate, that is a heat sink mass is increased, thereby increasing a thermal capacity of the power transmission device (please see FIG. 4).
In addition, two housings and two clutch hubs 210 are integrally formed with each other in the conventional power transmission device 200. That is, a cylinder having a suitable diameter is manufactured and the housing is then manufactured by cutting an inside of the cylinder. Therefore, manufacturing cost of the housing is very high.
In addition, when the power transmission device 200 is released, the operation oil may not be exhausted from the piston chamber 220 completely and remain in the piston chamber 220 due to centrifugal force. In order to exhaust the operation oil remaining in the piston chamber 220 completely, a balance chamber should be formed in the piston 230 to supply a balance hydraulic pressure. The conventional power transmission device 200, however, does not include the balance chamber.
Recently, research to increase the magnitude of the axial force applied to the piston while reducing the magnitude of the operating hydraulic pressure in the power transmission device has actively been in progress. Accordingly, a technique for mounting two or more pistons in one power transmission device has been undergoing development.
As the number of pistons is increased, the piston area to which the operating hydraulic pressure is applied is expanded, thereby increasing the magnitude of the axial force applied to the pistons and increasing torque capacity of the power transmission device. However, if the magnitude of the axial force applied to the pistons is increased, a return spring force that is applied against the axial force also needs to be increased. Particularly, it is very important to appropriately determine the return spring force in order to properly operate the power transmission device including two or more pistons. However, the research on determination of the return spring force has not yet progressed.
Because the power transmission device provided with two or more pistons increases an axial force applied onto the friction disks, pressure applied onto the friction disks needs to be distributed evenly to increase lifespan of the friction disks. In addition, research on improving a thermal capacity of the power transmission device by increasing a heat sink mass of the friction disks is necessary.